Motor operated valve combinations or valves are commonly used in nuclear and other power plants to perform routine opening and closing of water and steam lines and, more importantly, for those valves designated as safety related, to perform such openings or closings to assure prompt, safe shutdown of the plant under accident or other emergency or abnormal operating conditions. For those valves of this class whose safety function is to close a water, steam or other line, it is important that the valve achieve full or complete closure when called upon to do so. Most motor operated valves in use today are controlled by a torque switch which shuts off or stops the operation of the operator motor when a predetermined valve actuator torque limit is reached and a spring pack or other component moves in response to actuator torque to trip or actuate the torque switch. Since the torque switch displacement is approximately proportional to the actuator torque above some preload torque, and since the actuator torque is approximately proportional to the stem thrust, the torque switch displacement set point can be and generally is used to turn off the motor at approximately a value sufficient to fully close the valve and provide some additional margin to both help insure against leakage and account for possible degradations and non repeatabilities that could reduce that margin.
A certain amount of stem thrust is required for a motor operated valve (MOV) to close against differential pressure. The required thrust can be calculated as the sum of the packing friction force, the stem rejection force, and the differential pressure force. The packing friction force is the force required to push the valve stem through the packing which has been compressed against the stem to prevent leakage. The stem rejection force is the force of the internal pressure trying to spit out the valve stem. It is equal to the internal line pressure times the cross sectional area of the stem. The differential pressure force is the friction force opposing the inward movement of the valve disc as it is pushed in by the valve stem to close off the opening of the valve. It is equal to the differential pressure across the valve times the effective area of the valve disc against the downstream seat, this coefficient of friction is often referred to as the valve factor. The valve factor is the least predictable, and possibly least constant factor for required thrust.
A control switch, typically the torque switch, is adjusted to trip and thereby turn off the operator motor at a stem thrust level which is generally higher than the computed required closure thrust so as to provide a comfortable margin to account for unpredictable or changeable factors such as the valve factor in the required thrust, and such as stem lubrication degradation which can reduce the achieved thrust for the given amount of achieved torque. Typically a thrust verification test is run at the valve to determine the actual thrust applied to the valve stem at control switch trip (CST), usually torque switch trip, making sure the actual thrust exceeds the computed required closure thrust by a comfortable margin. The at-the-valve test is repeated periodically to insure that degradations such as lubrication degradation have not significantly reduced the level of thrust being developed at CST. Lubrication degradation increases the friction between the threads of the valve stem and the threads of the stem nut that drives the valve stem. This increased friction is what causes lower thrust for the given amount of torque.
Co-pending Parent U.S. patent application Ser. No. 08/071,421, filed Jun. 3, 1993, describes a method for remotely quantifying lubrication degradation so that during times when plant maintenance personnel cannot be at the valve during plant operation (common in nuclear power plants), proper lubrication may be remotely verified, and if improper lubrication should be detected, operations may be halted so that the necessary relubrication can take place. The parent patent application also describes a remote means for approximating thrust differences above or after the onset of wedging including the reverification of thrust margins, defined there as the difference between the thrust at the onset of wedging and thrust at CST. Valuable as such knowledge is, the methodology described in the parent application does not address how to remotely approximate thrust differences below as well as above the onset of wedging, nor how to approximate absolute values of thrust both below and above the onset of wedging. The methods to accomplish these results are the subject of the present invention.
The parent patent application recognizes and makes use of the fact that the time rate of thrust buildup after the onset of wedging remains constant independent of lubrication degradation. The present invention makes use of that fact as well as the fact that as the motor operated valve actuates, the ratio of change in thrust to change in power remains constant before and after the onset of wedging for a given lubrication condition.